In the past decade, monitoring of the electrical characteristics of gas flowing through a jet engine has been studied as a possible indication of engine deterioration. Apparatus disclosed in U.S. Pat. No. 3,775,763 utilizes an electrostatic probe positioned in the exhaust of the jet engine, such as through the tail pipe wall. Abnormal conditions were thought to be coupled with small particles striking the probe and causing spikes of ion current of a relatively large magnitude. Subsequently, it was theorized that the signals did not result from individual particles of metal hitting the probe, but rather that the signals were indicative of Trichel pulses (a form of repetitive corona discharge) created by high potential pockets of excess charge. This is as reported by Couch, R.P.: "Detecting Abnormal Turbine Engine Deterioration Using Electrostatic Methods", Journal of Aircraft, Vol. 15, October 1978,pp 692-695. A probe set including circular insulated segments within the gas turbine engine tail pipe and a triangle of wire extending through the tail pipe exhaust gas path were developed. With these probes, a normalized count of probe current (or voltage developed across an impedance) in excess of a threshold magnitude over a period of time definitely correlated with impending engine component malfunctions or severe deterioration. As reported in the aforementioned article, however, the use of normalized counts of large magnitude signals from the ring and grid probe was thought to provide reliable prediction of only two out of three gas-path failures, at best, without any distinction between possible causes thereof.
The limited results achievable with the grid and ring probe counting method described in the aforementioned article is due in part to the fact that the very high noise levels on the probes preclude use of a sufficiently low threshold to be sensitive to ionic phenomenon of lower magnitude. Additionally, the excessive noise masks the characteristics of ion phenomena, other than the occurrence of extremely large signals. In order to improve the reliability of prediction of an impending failure, and to discriminate, with a useful degree of success, between types of impending failure, much improvement in signal-to-noise ratio of the sensing probes is required.
Additional details of the segmented ring reported in the article can be found in Appendix C of Baker, F. L., "Electronic Analysis of Electrostatic Pulses to Detect Imminent Jet Engine Gas-Path Failure", Air Force Institute of Technology Thesis AFIT/GE/EE/77-7, December 1977 (Defense Documentation Center No. AD-AO56515). On the one hand, it is believed that the prior tail pipe semi-continuous ring has a poor signal response because it is disposed adjacent a large ground plane, namely, the tail pipe of the engine. Additionally, the semi-continuous ring has connecting segments outside of the ground plane that are responsive to both electrostatic and electromagnetic influences not within the gas path of the engine. It can therefore be understood that the signal-to-noise ratio of such a probe must inherently be poor.
Both the semi-continuous ring and the triangular grid probe are additionally noisy because of the large areas thereof that are subjected to material flowing through the gas path; in other words, a significant portion of material flowing through the gas path can contact these probes. This is particularly true of the triangular grid probe. The direct contact of these probes by particles in the gas path, in addition to electrostatic charge induced thereon, renders these probes inherently noisy with poor signal-to-noise differentiation, and they develop net current flow due to charges impinging thereon.
A recent innovation in the control of internal combustion engines, is described in a commonly owned copending U.S. patent application entitled "Method and Apparatus for Indicating an Operating Characteristic of an Internal Combustion Engine" filed contemporaneously herewith by Bullis et al, Ser. No. 432,501. Therein, the electrostatic charge of pulses of reciprocating, internal combustion engine exhaust provide an indication of solid carbon content, and therefore of the relative completeness of combustion in, and timing of, the engine. However, it has been found that use of a common, conductive shaft as a probe results in a very high DC current level and noise which tend to mask the pulses which are desired to be detected.
In high performance military gas turbine engines, the inclusion of an afterburner would render the aforementioned grid and ring probes useless, since they would be totally burned away. Additionally, probes disposed at the end of a gas turbine engine core are incapable of monitoring events within the afterburner itself. Thus, probes known to prior art are not useful for monitoring performance of the afterburner in military engines.
In many instances, it is desirable to employ electrostatic diagnostics with respect to a combustion engine without first having to modify the engine (such as the installation of the grid or rings as described in the aforementioned article).